Field of the Invention
The present invention relates to a control device for rotating electric machine, and in particular, a control device for a wound-field-type AC rotating electric machine which is mainly mounted on a vehicle, is connected to an internal combustion engine and a battery, operates as an electric motor for starting and assisting the internal combustion engine, operates as an electric generator for charging the battery, and has an armature winding and a field winding.
Background Art
In recent years, in view of requirement for improvement in fuel efficiency of a vehicle and environmental conservation, a vehicle that has an engine and another power source, e.g., a rotating electric machine, and which is called a hybrid vehicle, has been developed and put into practical use. In such a vehicle, it is necessary to selectively use the engine and the other power source and perform proper distribution therebetween, in accordance with the running condition.
For example, in order to suppress unnecessary fuel consumption due to idling of the engine, idle stop is adopted in many vehicles, in which the internal combustion engine is stopped when the vehicle has stopped due to a traffic light, traffic congestion, or the like, and thereafter, when the driver's intension to start to move the vehicle is detected on the basis of steering operation, release of the brake, or the like, the rotating electric machine restarts the internal combustion engine.
The rotating electric machine mounted on such a vehicle is connected to the engine, and operates with torque allowed to be transferred therebetween. Therefore, along with variation in the rotation speed of the engine, the rotation speed of the rotating electric machine also greatly varies, and a required operation rotation speed range is wide. Therefore, instead of a permanent magnet type in which a magnet is embedded in a rotor, a wound-field-type rotating electric machine is widely employed for which induced voltage can be easily control in accordance with the rotation speed.
In order to adapt to the idle stop described above, the rotating electric machine needs to be able to output great torque in a low-speed state when the internal combustion engine is restarted, and thus the battery voltage as a power supply and the operation voltage of the rotating electric machine are designed to be high, to achieve output enhancement.
In the case of using, as an electric generator, the rotating electric machine with the battery voltage designed to be high, in a low-speed region, induced voltage that can be generated by the rotating electric machine is insufficient for the battery voltage, and the battery cannot be charged. In such a state, it is necessary to charge the battery after stepping up the electric generation voltage while using an inverter as a step-up chopper by performing switching for the inverter (hereinafter, such a step-up electric generation method is referred to as an inverter electric generation mode). However, in general, in the inverter electric generation mode, the number of times of inverter switching increases, and the efficiency tends to deteriorate due to switching loss. Therefore, in a high-speed region in which induced voltage that can charge the battery can be obtained without being stepped up, it is desirable to obtain target electric generation voltage by performing rectification by a diode and controlling the energization amount of field current without performing inverter switching (hereinafter, such an electric generation method is referred to as an alternator electric generation mode). As described above, in the case where the battery voltage and the operation voltage of the rotating electric machine are designed to be high, an appropriate electric generation method differs depending on the driving condition of the vehicle, and therefore it is necessary to switch between these two electric generation modes in accordance with the driving condition.
As a method for switching between the inverter electric generation mode and the alternator electric generation mode, Patent Document 1 shown below discloses a feature of performing the switching in accordance with the rotation speed. That is, the rotation speed at which the electric generation mode is to be switched is controlled in accordance with the electric load amount of the rotating electric machine, and hysteresis is provided to the rotation speed at which the switching is to be performed, thereby reducing the frequency of occurrence of switching of the electric generation mode and reducing shock due to rotation variation.